Cannabis and Cancer Drugs Interact in Unexpected Ways, Study Finds

People with cancer often use cannabis for easing symptoms such as pain and nausea. But a new study shows that cannabis can interact with other drugs they take in potentially serious ways.

The researchers surveyed people with cancer who were receiving treatment at a cancer center about what medications they take. The survey asked about prescription medications, over-the-counter drugs, supplements, and cannabis.

Out of 313 participants, 61 of them were actively using cannabis. The researchers checked a database to determine potential interactions between cannabis and all of the medications these 61 participants took. They found 412 potential interactions.

Of the potential cannabis-medication interactions, 71% were with prescription medications not being used to treat cancer, 17% with over-the-counter-medications, and 10% with anticancer treatments. Eighty-seven percent of these had a risk of a moderate interaction, and 12% had a risk of a major interaction.

The most common cannabis-related potential medication interaction was with acetaminophen, an over-the-counter drug used for pain relief. The next most common was dexamethasone, a prescription medication used to relieve inflammation. The third most common was ondansetron, a prescription medication used to prevent nausea and vomiting.

The most common interactions between cannabis and chemotherapy drugs were with Taxol (paclitaxel), which is used to treat multiple types of cancer, including breast cancer. Taxol has the potential for major interactions with cannabis, meaning that marijuana could strongly affect either the effectiveness or side effects of Taxol. The second most common was Adriamycin (doxorubicin), which is also used to treat multiple types of cancer, including breast cancer. It also has a risk for major interactions with cannabis.

Of the 61 participants who were using cannabis, their average age was 58. The most common cancers they had were lung, breast, lymphoma, and colorectal. They mostly used cannabis through smoking or gummies, but some used it through a tincture, oil, or other liquid. The most common reasons they reported using cannabis were for insomnia, pain, and mood.

The researchers also found many potential interactions between supplements and medications the participants were taking — even more than they did between cannabis and medications. The researchers did not specify whether any of the patients experienced unusual side effects or other complications due to these potential interactions.

The research was presented at the International Conference of the Society for Integrative Oncology in October. 

This study is one of the first on cannabis-medication interactions in people with cancer, despite the fact that between 25% and 40% of people with cancer use cannabis. Because there are so few studies, the evidence of the cannabis-medication interactions is generally not very good, meaning some of the interactions identified in the study may not happen. But even if there’s only a chance that some of these medications could interact with cannabis, it’s important to talk with your medical care team before beginning to use cannabis.

Previous research has suggested that cannabis may decrease the effectiveness of immunotherapy for cancer treatment. Additionally, a 2022 review study found that cannabis may have severe interactions with the prescription medication warfarin, which is used to treat blood clots. It also found that cannabis may interact with buprenorphine, which is used to treat pain and opioid use disorder, and tacrolimus, which is used to prevent organ rejection after a transplant.